1. Field of the Invention
This invention relates generally to gas turbine control systems and more particularly to an improved control system and method of controlling fuel flow to a gas turbine to control the temperature thereof.
2. Description of the Prior Art
The present day high acceptance of large capacity gas turbine power plants for large peak load applications has made the reliability and economical operation of these power plants an important concern to the user. This is due in large part to the fact that these power plants are often located in remote areas and operate unattended and to assure higher turbine reliability and economical operation, turbine temperature regulation must be accurate. Remote control is sometimes provided by a telemetry communication link.
Traditionally, the operating temperature of a gas turbine has been controlled by regulating either the firing or exhaust temperature of the turbine. Regardless of which is to be regulated, multiple thermocouples or other temperature sensing devices are employed for sensing the turbine temperature. In most all turbines, to measure the exhaust temperature, a plurality of thermocouples are placed at various points in the turbine exhaust plenum and the temperature signals generated by the thermocouples are averaged to calculate a value proportional to the average exhaust temperature.
In the prior art, this averaging has been done by electronic analog elements interconnected as a temperature regulating system to control the average exhaust temperature at some predetermined level.
The analog elements in temperature regulators, whether they be pure analog or a combination of digital and analog, present several problems:
1. A temperature reference set point or base temperature, usually an analog reference voltage, is employed in controllers for comparison with the average exhaust temperature to derive a fuel control signal for controlling fuel flow to the turbine and hence the operating temperature. It is well known in the art that these analog reference voltages are susceptible to drift with time and/or temperature, thus leading to inaccuracies in temperature control.
2. In digital controllers tolerance and drift errors are introduced into the temperature calculations by the analog-to-digital and digital-to-analog conversion circuitry. Also, round off and truncation in digital calculations can introduce errors or inaccuracies in the temperature calculation.
3. Additional inaccuracies are also introduced in the temperature calculation due to summing resistor tolerances at the inputs to amplifiers. Inaccuracies are also introduced by various buffer amplifiers which are used for signal conditioning of input and output signals of the system controller.
The above inaccuracies all contribute to a disagreement between the aforementioned temperature reference set point and the calculated average turbine exhaust temperature. This disagreement accordingly, leads to inaccurate turbine temperature control.
A disagreement between the temperature reference set point and the calculated or determined average turbine temperature can be detrimental to overall system operation. For example, the loss of accuracy of only a few degrees in turbine temperature can degrade both turbine life and fuel economy. Each of these can contribute to a lack of customer or user acceptance of a system.
It is, therefore, desirable to provide a control system and method for accurately controlling the operating temperature of a gas turbine which enhances turbine life and reduces the operating fuel costs of the turbine.